Liquid crystal displays are used as a display device which is the most important in the multimedia society in a wide range inclusive of from mobile phones to monitors for computer, monitors for laptop computer, and television receivers. For the liquid crystal displays, a lot of optical films for enhancing display characteristics are used. In particular, retardation films play a large role for enhancement of a contrast in the case of being viewed from the front or oblique direction, compensation of a color tone, or the like. In conventional retardation films, polycarbonates or cyclic polyolefins are used, and all of these polymers are a polymer having positive birefringence. Here, the terms “positive” and “negative” of birefringence are defined as follows.
The optical anisotropy of a polymer film having been subjected to molecular orientation by means of stretching or the like can be expressed by a refractive index ellipsoid shown in FIG. 1. Here, in the case of stretching a film, a refractive index in the fast axis direction within the film plane is designated as nx, a refractive index in the film in-plane direction orthogonal thereto is designated as ny, and a refractive index in the thickness direction of the film is designated as nz. Incidentally, the fast axis as referred to herein means an axis direction with a low refractive index within the film plane.
Then, the “negative birefringence” as referred to herein means that the stretching direction is the fast axis direction, and the “positive birefringence” as referred to herein means that a direction vertical to the stretching direction is the fast axis direction.
Namely, in uniaxial stretching of a polymer having negative birefringence, the refractive index in the stretching axis direction is small (fast axis: stretching direction), and in uniaxial stretching of a polymer having positive birefringence, the refractive index in the orthogonal axis direction to the stretching axis is small (fast axis: vertical direction to the stretching direction).
A lot of polymers have positive birefringence. While examples of a polymer having negative birefringence include an acrylic resin and polystyrene, the acrylic resin is small in retardation and insufficient in characteristics as a retardation film. The polystyrene involves a problem regarding stability of retardation such that because of a large photoelastic coefficient in a low temperature region, the retardation is changed by even a few stress; a problem in optical characteristics such that wavelength dependency of the retardation is large; and moreover, a problem in view of practical use such that heat resistance is low, and hence, the polystyrene is not used at present.
The wavelength dependency of the retardation as referred to herein means that the retardation changes depending upon the measuring wavelength and can be expressed as a ratio (R450/R550) between a retardation measured at a wavelength of 450 nm (R450) and a retardation measured at a wavelength of 550 nm (R550). In general, in polymers of an aromatic structure, a tendency that this (R450/R550) becomes large is strong, and a contrast or viewing angle characteristic in a short-wavelength region is lowered.
In stretched films of a polymer exhibiting negative birefringence, the refractive index in the thickness direction of the film is high, and a retardation film which has not been found so far is produced. Therefore, for example, such stretched films are useful as a retardation film for compensation of the viewing angle characteristic of a display such as a super-twisted nematic type liquid crystal display (STN-LCD), a vertical alignment type liquid crystal display (VA-LCD), an in-plane switching type liquid crystal display (IPS-LCD), a reflection type liquid crystal display (reflection type LCD), etc., or a viewing angle compensation film of a polarizing plate, and requirements of the marketplace for retardation films having negative birefringence are strong.
On the other hand, there are proposed manufacturing methods of a film with an increased refractive index in the thickness direction of the film using a polymer having positive birefringence. One of them is a treatment method in which a heat-shrinkable film is adhered onto one or both surfaces of the polymer film, the resulting laminate is subjected to a heat-stretching treatment, and a shrinkage force is applied in the film thickness direction of the polymer film (see, for example, Patent Documents 1 to 3). In addition, there is proposed a method in which a polymer film is uniaxially stretched within the plane while impressing an electric field thereto (see, for example, Patent Document 4).
Besides, there is proposed a retardation film composed of fine particles having negative optical anisotropy and a transparent polymer (see, for example, Patent Document 5).
But, the methods proposed in Patent Documents 1 to 4 involve such a problem that productivity is inferior because the manufacturing step is very complicated. In addition, control of uniformity of the retardation, or the like is remarkably difficult as compared with the conventional control by stretching.
In the case of using a polycarbonate as a base film, a photoelastic coefficient at room temperature is large, and the retardation is changed even by a few stress, so that there is involved a problem regarding stability of retardation. Furthermore, there is involved such a problem that the wavelength dependency of the retardation is large.
The retardation film obtained in Patent Document 5 is a retardation film exhibiting negative birefringence by the addition of fine particles having negative optical anisotropy, and from the viewpoints of simplification and economy of the manufacturing method, a retardation film which does not require the addition of fine particles is demanded.
In addition, there is proposed a film composed of a fumaric acid diester based resin (see, for example, Patent Document 6). While the optically compensatory film obtained in Patent Document 6 has an out-of-plane retardation to some extent, a film having a high out-of-plane retardation even in a thin film is demanded.
Patent Document 7 discloses a fumaric acid diester copolymer having a storage elastic modulus (E′), as determined by the dynamic viscoelasticity measurement at 200° C. at a frequency of 10 Hz, of 2.0×107 Pa or more and exemplifies a copolymer of diisopropyl fumarate and di-n-butyl fumarate or bis(2-ethylhexyl) fumarate. Patent Document 7 is a patent application which is concerned with a fumaric acid diester copolymer having excellent transparency, heat resistance and mechanical characteristics and an optical film composed of the subject copolymer, in particular a plastic film substrate. However, Patent Document 7 does not provide a description regarding the retardation, and a resin with higher retardation revelation properties is demanded as the resin for retardation film.